Spinal fusion is a common surgery for the treatment of various spinal pathologies. During a spinal fusion, two or more vertebrae are fused together in order to eliminate abnormal motion caused by various pathologies (e.g., degenerative disc). Supplementary bone tissue, either from the patient or a donor, is used in conjunction with the patient′ natural bone growth processes, to fuse the vertebrae.
Improvements in spinal fusion operations were achieved with the introduction of internal fixation devices, which are used as an adjunct to spinal fusion. The fixation device contributes to the stabilization and immobilization of the treated spinal segment, thereby enhancing fusion and reducing pain. One such device is a construct of pedicle screws and rods: two screws are placed into the pedicles of each treated vertebrae, and rods are used to longitudinally connect the screws, using locking elements.
Pedicle screw systems, with or without fusion, are used, for example, for stabilizing broken vertebrae until healing (fracture union), oncologic cases, and treatment of vertebra abnormal curvatures, such as scoliosis and kyphosis. Also, dynamic stabilization method, using pedicle screw system without spinal fusion, is also performed nowadays.
U.S. Pat. No. 5,683,392 to Richelsoph et al. discloses “A locking mechanism for locking a rod to a bone member. The locking mechanism includes a bone fixation member for attachment to the bone member, the bone fixation member having a spherical portion; an inner housing member having a channel for receiving the rod and having a spherical portion for engaging the spherical portion of the bone fixation member; and an outer housing member for locking the inner housing member to the rod and the spherical portion of the bone fixation member.”
Normally, the pedicle screw-rod constructs are made of metal, such as titanium and stainless steel. Although metallic implants provide numerous advantages, they also have a few drawbacks. Metals obstruct visualization of the implant and surrounding tissue upon using fluoroscopy, CT and MR imaging. It is noted, that about 20%-30% of the patients continue to suffer following surgery with intra-pedicular implants. As the pedicle screws are located in adjacent to the spinal cord and nerves, such imaging means are highly important for follow-up evaluation, including for identifying screws exact location and determining whether the screws are the cause for pain. This problem also exist in cervical surgeries, where, for example, metal plates and screws are used for stabilizing cervical vertebrae with various pathologies (such as degenerative disc, fracture, tumor, stenosis), normally during fusion surgeries. Also, in oncology cases, evaluation of tumor progress may be limited due to artifacts produced by the metallic implants. Furthermore, metallic implants interfere with radiotherapy given to oncological patients. The relatively large electronic mass and the scattering phenomena reduce the radiation effectiveness and necessitate radiation in higher doses, that further provoke side effects on surrounding tissue.
In addition, metal construction normally provides adequate bending and torsion strength, thus reducing problems associated with implant fracture. However, the rigid metal implant, having different elasticity than that of the bone, may contribute to stress shielding phenomena. Furthermore, metals such as stainless steel may cause biocompatibility problems related to corrosion and sensitization reaction (mainly due to allergy to nickel). In addition, a resistance of metals to fatigue loads may be poorer than a resistance of some composite materials to a similar fatigue load.
Non-metal, composite material, spinal bone implants are currently available on the market, for example cage and vertebral body replacement devices made of carbon-polyetheretherketone (PEEK). Lumbar and/or cervical cages are also produced from PEEK, carbon fiber reinforced polymer or carbon.
Metal pedicle screw systems with rods made of polymer material (e.g., PEEK) are also market available. Yet, those devices provide for lower stiffness compared to metal devices and are used in limited indications. In addition, metal pedicle screw systems with rods or plates made of carbon fiber reinforced polymer (e.g., PEEK or PEKEKK) are also available.
Composite material bone implants, made of, for example, carbon fiber-reinforced PEEK, are also used for other bone applications, such as intramedullary nails and bone plates (CarboFix Orthopedics Ltd.).